Urban geology: The Boxtel wall game
Stephen K Donovan (The Netherlands)
The Netherlands is a land of museums, approximately 1,200 of them in a country the size of southeast England. Although the major cities have an ample supply – about 30 in Amsterdam, for example – there are many and varied museums dotted throughout the country. (I remember, in 2003, being driven to Arnhem and seeing a German Panther tank parked outside a small military museum – be ready for the unexpected.) For the geologist, one of the gems is Het Oertijdmuseum (= The Prehistoric Times Museum; formerly De Groene Poort) in Boxtel, in the province of Noord Brabant, north-north-west of Eindhoven. As may be deduced from Fig. 1, the museum has a specialist collection of dinosaurs and other saurian – replicas in the gardens around the main building and mounted skeletons inside.
I am a walker and I prefer to saunter from the station through the attractive town of Boxtel to Het Oertijdmuseum rather than take a bus. The walk is a long 30 minutes. As you near the museum, the route passes a most extraordinary building, Bosscheweg 107, ‘Den Daalder’. This appears to be an entirely conventional office block until you reach the end closest to the museum, when all is revealed – an artificial slope of rocks almost half the height of the building (Fig. 2A). This is a wall game (Robinson, 1996; Donovan, 2017) on a grand scale.
The slope itself has an appearance like a scree slope on the side of a hill or face of a quarry, but the similarity is no more than superficial. A scree slope in nature will likely be composed of only one or a few rock lithologies; the clasts will be angular; and there will be sorting under the influence of gravity and surface run-off of rain. At ‘Den Daalder’, there is a diversity of rock types, certainly more than figured in this article (Fig. 2B-F); clasts are commonly more or less rounded; and no sorting is apparent, cobbles maintaining a broadly uniform size range throughout. The rounding of clasts indicates that they were collected from the bed(s) of a river or rivers.
A: A general view of the ‘wall’; note gabions below the floor of offices at the top. Why the cobbles of the wall are vegetated except for the lowermost 1m or so is unknown.
B: Permo-Triassic coarse-grained sandstone. A well-rounded clast of cross-bedded sandstone, cross-cut by thin veins of quartz.
C: Permo-Triassic pebble conglomerate. This is notably more spherical than coeval clasts (Fig. 2B and D), possibly a feature determined by its lack of internal bedding. Clasts include quartz (white), probable basalt (grey and black) and what may be an intra-formational rip-up clast (red, left).
D: Permo-Triassic interbedded coarse-grained sandstone and pebble conglomerate.
E: An angular cobble of vein quartz.
F: A rounded cobble of basalt, which has been veined by quartz in multiple directions.
The top of the incline is angled, sloping to the left (Fig. 2A). This ‘exposes’ what looks superficially like an old cliffline, but what is actually a face of gabion baskets filled with clasts, a common feature of the Dutch townscape (Donovan, 2018a, b), but in Boxtel in an unusual situation. Above the gabions, the topmost floor of the building is prominent, looking like a development peeping over a cliff edge. The whole effect is distinctive.
Common clast types are coarse-grained sandstones and pebble conglomerates, red to purple in colour, and most probably derived from ‘red bed’ Permo-Triassic deposits – what in Britain would be called the New Red Sandstone (Fig. 2B-D). Coarse-grained sandstones may be cross-bedded and interbedded with pebble conglomerates (Fig. 2B, D). Other, coarser grained conglomerates are massive and no bedding is apparent (Fig. 2C). The clasts of all conglomerates are a mixture of lithologies (= polymict), commonly (vein?) quartz pebbles, but also including several rock types.
Bright clasts of vein quartz are more blocky than rounded, presumably a feature determined by the hardness of the mineral. The veins are broad and the flattened sides (Fig. 2E) are presumably defined by the walls of the vein.
Other rock clasts are varied. The basalt cobble (Fig. 2F) is strongly veined in multiple directions, including short, parallel tension gashes in the centre. In contrast, quartz veins in sandstone cobbles are simpler (Fig. 2B).
Despite the size of this wall game, I am not advocating clambering all over it – it is private property and much can be gleaned at pavement level. Perhaps a pair of binoculars would help. Robinson (1996, p. 164) recognised two aspects that should be addressed in any wall game:
- One is to identify the rock types, and their variety, colour, texture and durability. Such identifications take into account features such as sedimentary structures (Fig. 2B-D) and fossils (sadly, I identified none).
- Secondly, the quality of the stone for building is important – is it still fresh or badly weathered?
All of the clasts that I saw at ‘Den Daalder’ were fresh and solid, as would be expected from such river-transported clasts. Any weathered crusts, or jointed or faulted clasts would be broken down by the constant impacts associated with energetic river transport.
References
Donovan, S.K. 2017. Urban geology: the Worsley Park wall game, Manchester. Deposits, 52: 12-14.
Donovan, S.K. 2018a. Urban geology: gabions in the Dutch townscape. Deposits, 53: 14-16.
Donovan, S.K. 2018b. Urban geology: a failed example of gabions as false urban geology from the Netherlands. Deposits, 55: 10-11.
Robinson, E. 1996. A version of ‘The Wall Game’ in Battersea Park. In Bennett, M.R., Doyle, P., Larwood, J.G. & Prosser, C.D. (eds), Geology on your Doorstep: The role of urban geology in Earth Heritage Conservation: 163-170. Geological Society, London.